0.17.0 and 0.16.6 releases

[davidhewitt]

In #2481 we agreed that we'll need to have a "soundness" release 0.16.6 (with drawbacks) as well as a new 0.17.0 release ASAP which offers a fixed PyCapsule API.

I think this is everything which should happen for us to be ready to release:

• [x] Finish new PyCapsule API #2485
• [x] Write 0.16.6 patch for PyCapsule as per https://github.com/PyO3/pyo3/pull/2481#issuecomment-1167703814
• [ ] Get pyo3-ffi-check CI green (needs some fixes for PyPy and 3.11 beta)
• [ ] Finish #2477
• [ ] Implement https://github.com/PyO3/pyo3/issues/2392#issuecomment-1146697738
• [ ] Fix for #2501
• [x] Finish #2283

There are also a few other open PRs & issues I've left in the 0.17 milestone. It's fine for those to also be completed while we're tidying the release.